Resistant hypertension refers to patients with difficult-to-treat hypertension, generally defined as needing 3 or more blood pressure medications. It is a common medical disorder, affecting approximately 7-10 million Americans. Within this group of patients with resistant hypertension is a proportion of patients failing antihypertensive treatment in spite of maximum therapy. We propose referring to this antihypertensive treatment failure as refractory hypertension. In a preliminary analysis of our clinical experience over the last several years, we found that the prevalence of refractory hypertension was approximately 10% of patients initially referred for resistant hypertension. If generally true, this would translate into about 1 million Americans. Refractory hypertension has not been previously described. The current application is designed to test hypothesized mechanisms of antihypertensive treatment failure. In our initial analysis of this group of patients, we found evidence of increased sympathetic activity as an important contributing cause antihypertensive treatment failure. This hypothesized excess in adrenergic activity is suggested by higher resting heart rates and by higher levels of urinary norepinephrine levels compared to patients with controlled resistant hypertension. We will test for heightened sympathetic tone in this group of patients by prospectively comparing plasma norepinephrine levels in patients with refractory hypertension to patients with controlled resistant hypertension. We will also determine the antihypertensive benefit of blocking the effects of sympathetic activity with an agent that provides both alpha and beta adrenergic receptor blockade. Persistent intravascular fluid retention has been shown by us and other investigators to be a common cause of resistant hypertension. To what extent refractory hypertension is volume dependent is unknown. In a second series of experiments, we will test the hypothesis that persistent fluid retention contributes importantly to the development of refractory hypertension by comparing cardiac volumes, as measured by magnetic resonance imaging, in patients with refractory versus controlled resistant hypertension. In a second experiment, will compare the antihypertensive benefit of dietary salt restriction in patients with refractory hypertension versus control patients, to determine to what extent salt sensitivity contributes to antihypertensive treatment failure. Lastly, obstructive sleep apnea is extremely common in patients with resistant hypertension and is thought to contribute to difficulty controlling blood pressure. The role that untreated sleep apnea plays in causing refractory hypertension is unknown. The third objective of this application is to determine the prevalence of moderate/severe obstructive sleep apnea in patients with refractory hypertension compared to patients with controlled resistant hypertension. If poorly treated obstructive sleep apnea is common in patients with refractory hypertension, it will indicate an important and potentially reversible cause of antihypertensive treatment failure.